There have been a number of ways that the stability have been provided for in marine structures.
Floating facilities either ship-shape or column stabilised, achieve stability through changing buoyancy by part of their body coming in or out of water. It is because of this requirement that a large part of their body providing buoyancy and stability is exposed to maximum environmental loading. PA1 Submarines achieve static stability by ballast control using external power of pumps to pump water in or out of the vessel as required. PA1 Non-floating structures are stabilised either by piling to the sea bed as foundation or by being attached to large weights known as gravity foundations. PA1 There are also tethered structures which have tethers stabilised by gravity or piling, acting against the buoyancy of a floating vessel and keeping the tethers always in tension. The stability of the vessel is either provided by the tension in the tethers or a combination of the tension as well as the buoyancy changes due to the vessel coming in or out of the water. PA1 Dynamic positioning systems which use trusters to resist the wave forces. These are also used to turn a ship around to face the waves. Turrets are required to allow the ships to turn around a moonpool housing pipes connected to the source of the minerals. PA1 Tensioners can be used but these have the expense of the foundation as well as the tensioners. PA1 (i) a first portion adapted to be connected to the marine body, said first portion defining a chamber adapted to contain a variable volume of compressed gas, the first portion being open to the water at or near its lowermost-in-use region to allow ingress of water to form a water pool inside the first portion, the surface of the water pool being in contact with the compressed gas above it; PA1 (ii) a second portion adapted to be moveable in relation to the first portion, said second portion also defining a chamber adapted to contain a variable volume of compressed gas: PA1 (a) providing one or more buoyancy assemblies as described above; PA1 (b) attaching the or each buoyancy assembly to a marine body; PA1 (c) providing control means adapted to control the operation of the or each buoyancy assembly such that the marine body is stabilised at the desired point of hydrostatic equilibrium or provided with the desired foundation stability.
Providing piled stabilised foundation is expensive, requiring specialised crane vessels, pile driving hammers and the expense of the piles. Gravity stabilised foundation require large, usually concrete, structures and expensive ballast systems. These structures need to be either externally stabilised for transportation and installation offshore or their stability element would need to be water surface piercing attracting environmental loads. Structures sitting on the sea bed fully submerged would need crane vessels to lower them down or raise them up and still require to be stabilised by a foundation.
Floating facilities which are utilised as offshore platforms for mineral production require to keep station whilst being connected to the source of the minerals. However, as they need to be surface piercing for their stability, they are at times subjected to severe environmental loads. In order to minimise their motions and for station keeping a number of facilities have been developed. These include:
In summary, the known technology is expensive to install and expensive to operate on a day-to-day basis. No one technology can provide buoyancy and stability when partially and fully submerged as well as providing foundation stability to a structure sitting on the sea bed. No one existing marine stabilising system has the facility to alter its dynamic characteristics to suit changes in the environmental loads. It is the object of the present invention to overcome some or all of these disadvantages.
This invention relates to marine bodies that can provide hydrostatic stability even when fully submerged, have the facility for altering its dynamic characteristics and provide sea bed foundations that minimise or eliminate the vertical loads acting on the sea bed.